I understand that ETTL means a pre flash is fired to determine the correct output the flash should emit.

Correct.

BUT i use both the 430II and the 600ex, and i never see a pre flash fire.

The pre-flash and the "real" flash are milliseconds apart. Virtually undetectable by the human eye. Subconsciously, some people can perceive the pre-flash and this is what causes them to blink. You could also take something like a DV camcorder and record a TTL flash firing and then watch each individual frame and you can see one frame with a splash of light and the next with a large burst of light. DV material is roughly 30-frames/second.

I ALWAYS use flash exposure compensation - like the freedom of adjusting the output myself. Does this automatically mean that a pre flash doesn't fire, because the output is never set to 0?

FEC has nothing to do with whether a pre-flash fires or not. If you are using TTL, FEC or not, a pre-flash fires.

I'm a little confused here!

Only insomuch as you believe you should be "seeing" the pre-flash. You won't.

Thank you for your response,

Another question is, the pre-flash is meant to determine and automatically set the output of the flash. But i don't see this happening, because like i've said i always adjust the flash output myself.